After hardening, the workpiece is hard but brittle. Martensite, which appears in steel during hardening, has a very high stress in it, so that the knife is not brittle, it is necessary to remove some of this stress....

How to choose a cooling medium? Again: expertise and experience or trial and error method. Based on the CTT (Continuous Cooling Transformation) diagram for a specific steel! https://en.wikipedia.org/wiki/Continuous_cooling_transformation
Fig. 1 CCT...

To what temperature I heat to, how long to keep the knife at this temperature: the manufacturer often presents it at a temperature scale but usually without the time of Austenitization! In what do I cool it in, how fast do I do it, are even harder questions. The manufacturer often provides...

Steel (iron alloy (Fe) with carbon (C)) without any special heat treatment, simply cooled after forging on the open air is soft. A knife made like this can bend in your hands. Before our ancestors had learned how to heat-treat steel, they forged it without any heat....

Another method to check the impact strength indirectly is the breaking-point test. Cutting the steel and hitting it with a hammer break the sample (e.g. knife) as in Izod's test. https://en.wikipedia.org/wiki/Izod_impact_strength_test When viewing a broken specimen under a magnifying glass, we...

The mythical toughness of steel for knives, everyone has heard about it, nobody has ever seen it. Impact strength is the material's resistance to cracking under sudden, rapid load / impact. This is the effort required to break a standardized sample, on a standard machine, under standardized...